This invention relates to a tightly sealed prismatic battery in which a non-circular spiral electrode unit is housed within a prismatic shaped can.
Increasing demand for portable office automation equipment and communication devices has resulted in an increasing requirement for nickel-cadmium and lithium ion rechargeable batteries as power sources, and in particular, for prismatic shaped batteries that can be efficiently mounted. In addition, since batteries are used as power sources for portable devices, it goes without saying that large battery capacity per unit volume is important. In other words, the advance of light-weight portability in recent years has brought about serious regard for high energy density characteristics.
Changing the can material from iron or stainless steel to lighter aluminum or reducing the can thickness are effective ways to make a battery light-weight. This is because the weight of the can is a large fraction of a battery's gross weight. However, when the can material is changed to aluminum or the can thickness is reduced, the strength of the can drops significantly. For example, since the longitudinal module of elasticity of aluminum is one third that of iron, the bending strength of an aluminum can is only one third that of an iron can with the same dimensions.
When can strength is reduced and the battery's internal pressure increases, can deformation can result. Internal pressure increases when gas is produced inside a tightly sealed battery. Abnormal battery conditions such as a short circuit, excessive current flow, or over-charging can result in gas production. A can with insufficient strength has the drawback that it can deform due to a rise in internal pressure. Can deformation can cause various problems. For example, large can deformation can result in breaking open the battery and damaging the electrical device in which it is installed. In a tightly sealed battery which depends on electrical contact between the outer winding of an electrode unit and the can which confines it, can deformation can increase contact resistance by separating the can from the electrode resulting in increased internal battery resistance.
The technology for a novel battery can reinforcing structure is described by Japanese Non-examined Patent Publication No. 62-93854 issued April 30,1987. The tightly sealed battery described in this disclosure is reinforced by providing a region of thicker material on the battery can.
This thicker material can reinforce the can and reduce deformation. However, thicker material on the battery can increases the external dimensions of the battery in opposition to the objective of miniaturization. The present invention was developed to further solve these problems. It is thus a primary object of the present invention to provide a tightly sealed shaped battery that can effectively prevent deformation of the can without increasing its dimensions.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.